A wavelength division multiplexing (WDM) optical communication technology is one of the technologies that have been currently and mostly applied to backbone network and metro network and refers to a technology for performing wavelength division multiplexing (WDM) on an optical line including optical fibers and transmitting a plurality of high speed signals. A WDM transport network requires an optical add/drop multiplexer (OADM) function of selectively branching/combining some wavelengths without photoelectric conversion and passing other some wavelengths. OADM may connect intermediate nodes present on a transmission line in wavelength units, thereby expanding network connectivity and enhancing efficiency. Reconfigured OADM (ROADM) is advantageous in that branched/combined wavelengths may be reconfigured at a remote place and a wavelength connection state of an entire network may be effectively reconfigured without an expert so as to flexibly handle a change in traffic state, thereby remarkably reducing network maintenance costs.
ROADM is largely classified and used into a switch based structure and a broadcast and select (BS) type structure. Recently, a method using the latter is more advantageous to accommodate a plurality of nodes in respect to low path loss and thus is more preferred in a system. The BS type ROADM system is a main component for configuring a system by a light distributor, a wavelength multiplexer/demultiplexer, a variable optical attenuator (VOA), a tunable wavelength filter, and a tunable wavelength laser. In particular, a tunable transponder configured by integrating a tunable wavelength laser and a tunable wavelength filter provides a function of varying a wavelength at a remote plate and reconfiguring a network, and thus a stock burden of back-up optical components is reduced for a network operator to reduce time for network management and a random wavelength may be added/dropped for selection of wavelength to be added/dropped to effectively handle a change in traffic state. Accordingly, the tunable transponder is the most effective ROADM technology for reducing maintenance costs.
However, since the tunable wavelength filter technology is not enhanced and a tunable wavelength laser is very expensive, there are impediments to developing a tunable wavelength transponder.
With regard to a tunable wavelength filter, although an optical fiber Bragg grating-based filter has been currently developed, tunable wavelength response time is 5 seconds, which is very long and the price of the filter is also high, and thus, use of the tunable wavelength filter for a commercial system is low.
With regard to a tunable wavelength laser, although a laser using a distributed feedback (DFB) structure has also been developed and used, a tunable wavelength has a narrow range of 10 nm or less, and thus, the tunable wavelength laser is disadvantageous to use three to four sets of tunable wavelength DFB laser modules in order to support all wavelengths within a C-band (1535 nm to 1565 nm). In addition, a tunable wavelength transponder using a DFB laser uses an expensive light source and a multichannel transponder needs to be prepared for back-up. Thus, the tunable wavelength transponder is not an effective solution for reducing a stock burden to a network operator.
Accordingly, in order to embody effective and economical tunable wavelength transponder for a ROADM system, there is a need to develop an external-cavity type tunable wavelength light source using a tunable wavelength filter for varying all required wavelengths of a WDM band (e.g., C-band) by one module and a tunable wavelength filter providing a wideband tunable wavelength function.
Examples of the tunable wavelength filter technology include a tunable Fabry-Perot filter, a micro machined device, a Mach-Zehnder interferometer, fiber Bragg gratings, acousto-optic tunable filters, electro-optic tunable filters, arrayed waveguide grating (AWG), an active filter, ring resonator tunable filters, etc.
An optical waveguide type polymer tunable wavelength filter technology using Bragg gratings is disclosed in U.S. Pat. No. 6,303,040 (registered on Oct. 16, 2001, Title: Method of fabricating thermo optic tunable wavelength filter).
A conventional technology for a polymer optical waveguide type tunable wavelength filter refers to a technology for changing a refractive index of a medium using a thermo-optic effect and optionally reflecting or passing required specific wavelength of light and uses a heating element 13 (in general, a metallic thin film) for locally generating heat at an upper end of a polymer optical waveguide 12 in order to change an effective refractive index of the polymer optical waveguide 12 to vary an operating wavelength of a filter (refer to FIG. 5).
However, the conventional technology using a metallic heating element is disadvantageous in that a constant filter operating wavelength is not always provided regardless of an external environment because a relationship between a heating value generated from the metallic heating element and a required filter operating wavelength is changed according to an external environment when an external temperature changes.
Accordingly, a configuration for compensating for a temperature according to a change from the external environment needs to be used.
However, in general, a thermistor 11 as a general chip for measuring a temperature needs to be positioned on a surface of a wafer or next to the wafer that is spaced from a waveguide through which light passes due to the characteristics of the thermistor 11. Accordingly, a temperature change between the thermistor 11 and the waveguide that actually experiences temperature change occurs.
In this case, a temperature difference between a thermistor and a waveguide that directly experiences a temperature change that affects a laser wavelength may seriously and adversely affect the stability of a wavelength.